The major objective of this project is the study of mechanisms of host defense against virus-induced transplantable neoplasms. We are utilizing as a model the murine leukemia tumor FBL-3. This tumor, originally induced by Friend virus in a C57BL/6 mouse, has been found in vivo to undergo rapid and predictable changes in malignant potential. Ascites cells harvested at 7 days produce transient s.c. tumors that are rejected. Ascites tumor cells harvested late (14 days) produce progressive lethal tumors with widespread lymphogenous and visceral metastasis. Using these two cell populations and clones therefrom, we have compared the expression of membrane antigens, production of oncornavirus, and in vivo and in vitro immunogenicity. These studies are directed at gaining an understanding of the cellular mechanisms of altered behavior in vivo and defining the membrane structures required for successful immune recognition and elimination by the host. The primary immune response to the tumor cell inocula is currently being studied to define the early events in tumorigenesis that determine the ultimate behavior of the neoplasm.